Magnetic resonance spectroscopy (“MRS”) provides in vivo information regarding the concentration of specific metabolites (e.g., shown in parts per million, or ppm) in localized regions of the brain. Various challenges exist in MRS technology. Among these are SNR issues, relating to the very low concentration of metabolites as compared to water, and chemical shift displacements at higher magnetic fields when spatially-selective excitation is used.
In proton magnetic resonance imaging (“MRI”) (1H MRI), the image signal mainly relies on the proton of water, abundant in vivo. This hydrogen proton signal is the basis of MRI collection and without it, no MRI data can likely be collected. In contrast, in MRS data collection, special water suppression methods are used to suppress the proton signal from water to visualize the proton signals from the low-concentration metabolites, which range from 1 mM to 10 mM.
In image processing, the signal-to-noise ratio (“SNR”) of an image is usually defined as the ratio of the mean pixel value to the standard deviation of the pixel values. MRS can yield an inferior signal-to-noise ratio per unit time as compared with MRI because concentrations of brain metabolites may be of orders of magnitude lower as compared with tissue water. Several techniques have been developed to compensate for the low SNR of MRS.
To improve SNR, MRS voxels can be typically more than one thousand times larger than those of MRI, e.g., on the order of cubic centimeters rather than cubic millimeters, and acquisition times may be on the order of 100-1000 times longer, e.g., many minutes as compared with a few seconds. Substantial technical and methodological efforts have been dedicated to improve the SNR and reduce the acquisition time of MRS, especially with respect to proton (1H) variants, which may be prevalent because they can use the same hardware.
Another technique for improving SNR, for example, comprises multiplexing in space. Multiplexing in space may comprise increasing the spatial coverage from single- to simultaneous multi-voxel acquisition. Such MRS imaging (MRSI) can provide more spatial information at a similar SNR as determined by the voxel size and acquisition time. See, for example, NMR Chemical Shift Imaging in Three Dimensions, Brown T R et al., Proc Nat Acad USA 1982; 79:3523-3526; Spatially Resolved High Resolution Spectroscopy by “Four Dimensional” NMR, Maudsley et al., J Magn Reson 1983; 51:147-152; Short Echo Time Proton MR Spectroscopic Imaging, Posse et al., J Comput Assist Tomogr 1993; 17(1):1-14; Noise in MRI, Macovski A., Magn Reson Med 1996; 36(3):494-497, all incorporated herein by reference in their entireties. To reduce SNR loss from incomplete longitudinal relaxation, T1, the TR can be extended beyond the actual acquisition-cycle, TC, the time needed to prepare and acquire one transient, which may be determined primarily by the desired spectral resolution. Because such system “recovers” during the period (TR−TC), its duty cycle (“DC”) can be inherently suboptimal, e.g., <100%.
A further technique which may be applied to proton MR spectroscopic imaging (1H-MRSI) comprises multiplexing in time. This technique can maximize the amount of information obtained per unit time by increasing the DC, as described by Duyn et al. in Fast Proton Spectroscopic Imaging of Human Brain Using Multiple Spin-Echos, Magn Reson Med 1993; 30(4):409-414, incorporated herein by reference in its entirety. Multiplexing in time may comprise acquiring several TC-s for every TR, where each can be obtained from a different slice. Reference is made to Multisection Proton MR Spectroscopic Imaging of the Brain, Duyn et al., Radiology 1993; 30(4):409-414, incorporated herein by reference in its entirety. This technique can, in some situations, lead to a suboptimal SNR. For example, TR may be longer than the optimal, TRopt, in order to satisfy spatial coverage and spectral resolution requirements, which can lead to a suboptimal SNR.
A technique for 3D-multislab MRI comprises multiplexing in both space and time and is described by Goelman in Fast 3D T(2)-weighted MRI with Hadamard Encoding in the Slice Select Direction, Magn Reson Imaging 2000; 18(8);939-945, incorporated herein by reference in its entirety.
Another challenge arising in SNR technology is chemical shift displacements (“CSD”). Selective radio frequency (RF) pulses applied under a gradient can excite spins in different spatial bands during 1H-MRSI. Such bands can be displaced relative to each other by a distance, Δr, in the gradient direction, r (e.g., an X, Y or Z direction), as discussed by Kim et al. in High-field Magnetic Resonance Techniques for Brain Research, Curr Opin Neurobiol 2003; 13(5):612-619, incorporated herein by reference in its entirety. The displacement, which can be referred to as a “chemical shift artifact,” can depend on a resonance frequency difference of the spins, Δωi, and thus, e.g., on the magnetic field, B0.
Higher values of B0, therefore, can present conflicting requirements with respect to 1H-MRSI analyses. For example, providing a stronger Gr to maintain or reduce relative magnitude and direction of the CSD, Δr, for better localization accuracy can require a higher RF pulse bandwidth (“BW”) to retain a given field-of-view (“FOV”) and flip-angle. However, higher power which may be needed to produce the same RF field (B1) (e.g., the power required at 7 T can be twice that required at 4 T), may preclude any meaningful gradient increases. Consequently, while Δr can increase with B0, it may not be possible to significantly boost Gr to mitigate this effect, and often it can be made even weaker to accommodate lesser B1 values, which may exacerbate the displacement.
Thus, there may be a need for improved MRS techniques, and e.g., 1H-MRSI techniques, which may overcome some of the difficulties and limitations described herein above.
These and other objects, features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the appended claims.